Fixed bias regulator

Testing the Rod Coleman Fixed Regulator

As part of my 300B SE project design, I looked into various fixed bias arrangements and regulators. Rod Coleman has developed another fantastic circuit after the success of his DHT filament regulator which is now the preferred filament supply kit within the DIY audio community. After many years of refining the DC filament regulator, Rod came up with a clever design for fixed bias using the same concept: a gyrator and a temperature-compensated CCS. Instead of feeding a current through the DHT filaments, in this case the current is used to generate a clean bias voltage across a “bias resistor”. The bias resistor is bypassed by a capacitor as the high impedance loop formed by the regulator and the bias resistor is sensitive to pick up HF noise which could be amplified by our system.

The kit is of the same quality you would expect from Rod’s boards and very easy to build. It takes less than an hour to build the boards:

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814 SE A2 Amplifier

Goodbye 4-65a SE, at least for now

After enjoying the 4-65a SE amplifier for many months, I couldn’t resist myself from upgrading the output stage to the 814s. I just needed changing sockets and filament raw supply transformers to fit the requirements of this lovely transmitting valve. Needless to say, my recent tests on 814s were very encouraging. The 814 seemed to perform much better than the 4-65a in delivering 10W of class A2 sound at half the distortion levels. This to me, was only worth trying.

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Upgrading the DHT filament regulators

I upgraded the DHT filament regulators to version 4. Rod has released a new kit which introduces temperature compensation for high current filaments. This is a key feature in my 4-65a design and I shall explain why:

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4-65a SE Amplifier: testing filament regulators

The Rod Coleman Filament Regulators were already tested in the New Year. It seems a long time indeed. Here is a selection of pictures after fitting the regulators in their heat-sinks and doing a final test before assembly:

4-65a

Testing the 4-65a filament regulators

4-65a filaments are lovely

46 filament bias and the regulator

46 Driver regulator

26 DHT Preamp Gen2 by Rui Lourenço

Rui's 26 DHT preamp in operation — Rui’s 26 DHT preamp in operation

Here is a great post by Rui Lourenço who finished a sublime incarnation of the 26 DHT preamp Gen2. I hope you find this post as inspiring as delightful it is for me to see someone taking my 26 DHT preamp version to the next level of perfection. I think Rui’s pictures will speak for themselves showing the great craftsman skills and amount of effort and dedication put into this art work.

My Type 26 Tube Preamplifier adventure – by Rui Lourenço (Portugal)

I’ve started my interest in diy audio about 2 decades ago, basically at that time because I had no means of buying some of the fantastic equipment’s I saw in several Audio Shows, also because the bug was there, and some of my friends were doing it. Today, fortunately I have the means, but the bug was kept and personally I believe that good implemented diy projects can most of the times be several notches above commercial products, or otherwise you would have to spend some important thousands of Euros to acquire them at a similar performance level. You all agree that the personal filling of achievement vs your friends drop jaws when looking to some of the wonderful projects we find everywhere, is also very good.

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46 DHT driver final tests

Having built the 4P1L filament bias driver stage in a breadboard, I now have the sufficient voltage swing to drive the 46 to maximum sweep. In my 4-65a SE amp, a maximum of 200Vpp is required to drive the amp into class A2.

The following tests conditions were used:

4P1L first stage:
- DN2540 gyrator in mu follower output
- 220nF/450V Capacitor coupled into 46 driver
- Filament bias: 15 ohms, Vgk=-10V
- Vsupply=355V and Va0=210V
- Output set to about 30-32Vpp to drive 46 at 200Vpp
46 driver stage:
- IXYS 01N100 gyrator in mu follower output
- Load impedance is 100K (Pete Millett’s interface)
- Filament bias: 10 ohm / 100W Vgk=-17V
- Vsupply=355V and Va0=204-208V
- Output set to 200Vpp

I tested 28 valves. Just a few of my lot are NOS. The average THD was about 0.4-0.5% but a good selection of 8 valves (mainly Sylvania NOS) provided a consistent 0.18% THD:

Happy now with the initial tests and selection of 46 pairs for the amplifier, I can now continue with the build…

4-65a SE Amp: refining the 46 driver

I did some tests today and looked at minimising distortion of this 46 driver in filament bias and found that Va=230V (instead of 184V) to provide best performance:Filament bias resistor array is now laid out horizontally to improve the dissipation of heat.

Here is the performance (0.05% at 17Vrms) at maximum drive input from my audio test set:

4-65a SE Amp: testing the 46 driver

46 driver breadboarded. The mu-follower gyrator, the filament bias resistor array and the nice teflon UX5 socket from Jakeband. The filament bias resistor array is formed by 4 10Ω 20W dale wirewound resistors. These get very hot so probably need to think an alternative layout or further resistors in parallel:The performance is very good. I just picked up a random 46 from my stock and biased it at 204V (which is the operating point in my design) achieving less than 0.05% at 10Vrms. Need to re-run this test to see how will perform at 70Vrms:

Using my spice model created from a good 46 valve, THD should be around 0.15% at 200Vpp with a 100K load and performance is great to loads down to 100 ohms. Clearly the load in A2 will change from high impedance to some kΩ so this driver should maintain outstanding linearity all the way through:

4-65a SE Amp: filament regulators

Finished the 4-65a filament regulators. Filament noise is only 1.5 mVrms at full load (3.5A):

4-65a SE Amp: 46 Driver Gyrator

A day of PCB etching

After a lot of work today in designing many PCBs, I finally got a pair of mu-follower MOSFET gyrators for the 46 driver stage. The driver has to provide very low impedance to operate the 4-65a output valve in class A2. The gyrator in mu-follower configuration will enable the right bias point as the amplifier is DC coupled as well as maximum signal (and current in A2) with minimum distortion.

Many don’t like sand at all in their amplifiers. I have a lot of experience with gyrators and CCS loads in pre-amps and drivers as well. I have to say that with MOSFETs gyrators the sound is really nice. For an A2 driver, not many options are available and the gyrator is a great choice for this job.

I built two PCBs (one per channel) and the circuit is the classic depletion-mode MOSFET gyrator based on the high-voltage IXTP01N100D. I guess that a DN2540 should work as well here but I’ve been saving the IXYS for this occasion. The reference voltage for the anode bias point is provided by the CCS formed by M1 (LND150) which provides a higher impedance in AC improving the frequency response of the gyrator.

The 46 is operating in triode-mode and filament bias with a Rod Coleman filament regulator. R6 is approximately 1/gm and output voltage is set by P1 to achieve the 4-65a bias point as the amplifier has stacked power supplies given coupling is DC, so no capacitors in the path to the grid.

Next: some tests on these gyrators and the filament boards…